Compressed storage of data items

ABSTRACT

Storing of data items in a memory ( 31 ) is provided wherein the data items are divided into successive data pieces of decreasing significance, and the data pieces are stored in respective parts of the memory ( 31 ), and when applying a data piece to the memory ( 31 ) in case all candidate memory parts are assigned to other data pieces: if the significance of the applied data piece is lower than a lowest significance of the other data pieces, discarding the applied data piece; if the significance of the applied data piece in one of the candidate memory parts at expense of a given other data piece which has a lower significance that the significance of the applied data piece, wherein the significance value of a data piece is based on a total or absolute distortion value of the data item. Advantageous use of the invention is made in applications using a device of fixed storage capacity for storing a flexible number of compressible data items, such as video, images, audio, speech.

[0001] The invention relates to storing data items in a memory, toreading data items from a memory, and to a storage medium having storedthereon data items.

[0002] It is known to use a storage medium of fixed size to storemultiple objects which may be subjected to non-reversible (lossy)compression. Such applications include digital still cameras, whereimages are stored on a flash card, floppy disc or hard-drive. Further,video cameras are known in which multiple recordings are stored on ahard-drive, optical disc or tape. Also known are storage devices such asembedded memories, where it is desirable to minimize the total storagecapacity used while still retaining a best possible image quality.Sometimes, a limited number of settings is available to the user tochange the trade-off between quality and capacity. For example, videocan be recorded in either ‘standard play’ or ‘long play’ mode, orsnapshots can be taken at either ‘standard resolution’ or ‘highresolution’. Once a decision has been made to use a certain quality, thedecision cannot be changed afterwards, even in those cases where amplestorage capacity is still available.

[0003] Cormac Herley discloses in 6th International Conference on ImageProcessing (ICIP '99), vol. 3, Kobe, Japan, 24-28 October 1999, storageof digital camera images. The camera is designed to store a fixed numberof images, and a lossy rate controlled compression is used to ensurethat each image fits in the space allocated to it.

[0004] An object of the invention is to provide improved storage of dataitems. To this end, the invention provides a method of and anarrangement for storing data items in a memory, and a storage medium asdefined in the independent claims. Advantageous embodiments are definedin the dependent claims.

[0005] According to a first aspect of the invention, each data item isdivided into successive data pieces of decreasing significance, the datapieces are stored in respective parts of the memory, and when applying adata piece to the memory in case all candidate memory parts are assignedto other data pieces,

[0006] if the significance of the applied data piece is lower than alowest significance of the other data pieces, the applied data piece isdiscarded; and

[0007] if the significance of the applied data piece is higher than thelowest significance of the other data pieces, the applied data piece isstored in one of the candidate memory parts at expense of a given otherdata piece which as a lower significance than the significance of theapplied data piece, wherein the significance of each data piece is basedon a total or absolute distortion of the data item at that data piece.By taken into account total or absolute distortions, reduction ofquality will always affect a data item having a (near) highest quality.The total or absolute distortion is a measure of the absolute quality ofthe data item; a low distortion indicates a high quality and vice versa.

[0008] The significance associated with a given data piece may be basedon the total or absolute distortion of the data item up to and includingsaid data piece. In this embodiment, the data item having currently thehighest quality will be reduced in quality.

[0009] Advantageously, the significance associated with a given datapiece may be based on the total or absolute distortion of the data itemup to but not including said data piece. In this embodiment, the qualityof the data item after the data piece has been removed is taken intoaccount, resulting in a better guarantee that the minimum media itemquality is maximized. In other words, in this embodiment, the data itemwhich will have the highest quality after quality reduction will bereduced in quality.

[0010] In a preferred embodiment of the invention, a record is kept foreach data piece, the record comprising the significance of the datapiece and the data item to which the data piece belongs. Thesignificance of each data piece represents the total distortion of thedata item at that data piece. The record piece may be locallyincorporated in a memory part, e.g. together with the data piece towhich it belongs. Preferably, the record is kept in an auxiliary memorywherein each record further comprises a pointer which indicates aposition in a main memory in which the data piece is stored.

[0011] Whenever a new data item is to be stored, it is preferablycompressed by a scalable compression method in order to produce ascalable bit-stream. An advantageous method is described inWO01/17268-A1. Such a scalable compression method has the property thatthe resulting scalable bit-stream give a best possible reconstructionquality for the given data item, given the number of bits available.When the scalable bit-stream is not cut off, the data item is encoded(near) losslessly, i.e. at a quality such that the reproduction isperceptually indistinguishable from the original. Scalable codingmechanisms are known for video and audio. In a preferred embodiment, thescalable bit-stream is cut into data pieces of the same sizes as thememory parts. Next, for each of these data pieces, a significance iscalculated. Preferably, the significance is calculated as a distortionvalue, which indicates an improvement in perceptual significance. Thesignificance is used to compare the data pieces of the new item with thedata pieces that are already stored. It can easily be understood thatthe data pieces produced by splitting the scalable bit-stream have theproperty of decreasing significance, since the scalable coding methodsfirst product the most significant bits. The data pieces of a new itemare then compared with the data pieces already present in the memory.When the new data pieces have a higher significance, previously storeddata pieces are overwritten.

[0012] Although a scalable coding method is preferred, also ahierarchical coding method may be used.

[0013] The invention makes it feasible to store a variable number ofdata items in a fixed storage space. These data items are preferablymulti-media objects, comprising audio objects, video objects, graphicobjects etc. The data items are always stored with an optimum qualityfor a given amount of stored information. When an additional data itemhas to be stored, the amount of occupied storage of the already storeddata item(s) is reduced (resulting in a quality reduction) enough to fitin the new data item, such that all data items will be stored atapproximately the same quality.

[0014] Although in a preferred embodiment of the invention all memoryparts have the same size, that is not required. The size of memory partsis chosen such that the storage capacity of multiple memory parts isusually required to store a single data item at a high quality. Forexample, a number of 8 storage data pieces is practical for storing animage.

[0015] In a preferred embodiment of the invention, a user is offered thepossibility to store certain data items at higher quality than otherdata items.

[0016] These and other aspects of the invention will be apparent fromthe elucidated with reference to the accompanying drawings.

[0017] In the drawings:

[0018]FIG. 1 shows a schematic diagram of an arrangement wherein dataitems are stored according to the invention; and

[0019]FIG. 2 shows a more detailed diagram of a part of the arrangementas shown in FIG. 1.

[0020] The drawings only show those elements that are necessary tounderstand the invention.

[0021]FIG. 1 shows an arrangement according to the invention, comprisingan input unit 1, a scaleable coder 2, a memory 3, a scaleable decoder 4and on output unit 5. The input unit 1 may be of any kind to obtaindata, e.g. an antenna, a camera or a storage medium. The data may bedirectly furnished to the output unit 5. The output unit 5 may be anykind of output unit, e.g. an antenna, a display or a storage medium.Before the data is furnished to the memory 3, the scaleable coder 2processes the data to obtain scaleable bit-streams. The scaleablebit-streams are then furnished to the memory 3. For retrieving data fromthe memory 3, a scaleable decoder 4 is present which furnishes decodeddata to the output unit 5 when desired. The memory 3 comprises anauxiliary memory (AM) 30 and a main memory (MM) 31. If necessary, somecontrol or processing unit may be included in the memory 3 to controldata flows.

[0022] In FIG. 2, the auxiliary memory 30 and the main memory 31 areshown in more detail, with an exemplary content. The main memory 31 isdivided into N memory parts for storing N data pieces. In this exampleN=11. The auxiliary memory is typically smaller than the main memory andis used for administrative purposes. The auxiliary memory comprises Nrecords, each record comprising several fields. A first field is aPointer to Main Memory (P), which comprises a pointer to a location inthe main memory 31 that holds the data associated with a given record. Asecond field is an Object Identifier (I) that comprises informationdescribing the data item to which the data piece stored in the mainmemory belongs. The object identifier refers to, for example, an imageout of a set of images stored by a digital camera. In a practicalembodiment the object identifier is zero in case a memory part is notassigned to a data item, e.g. when the memory part is empty. A thirdfield comprises a Significance (S). The S filed gives a measure of thesignificance of the data piece stored in the main memory 31 to which therecord refers to. The entries in the significance fields are preferablynon-negative numbers. The auxiliary memory 30 preferably has theproperty that the records are sorted on the significance. It is alsopossible to sort the records in a different manner, e.g. on theidentifier to group all data pieces belonging to the same data item.Within the respective group of data pieces, the data pieces may besorted on significance.

[0023] To add a data item, the data item is coded in coder 2 to producea scaleable bit-stream, which is split in to data pieces. The pieces aresubsequently processed. The significance measure of each piece is firstcompared to that of the block (piece) with the lowest significancemeasure currently in the memory. If the significance of the new datapiece is lower, it is not stored in the main memory, i.e. it isdiscarded. When a first piece is not stored, the processing can bestopped because the further data pieces have lower significance than thecurrent data piece (which is a property of a scaleable codingmechanism). If the significance is higher, the new data piece is writtento the main memory 31 at the position of the current least significantdata piece (which is obtained from the last position of the auxiliarymemory in case the records are sorted on significance). Thereafter, thelast record of the auxiliary memory is replaced by the records data forthe new data piece and the records in the auxiliary memory 30 arere-ordered to restore the ordering on significance. It is advantageousto start processing with the most significant data piece of an item (andthen with the subsequent data pieces having lower significance), becausethis is the order in which the data pieces are produced by the scaleablecoder 2, and further because these data pieces are not overwritten byblocks belonging to the same item since these are less significant.

[0024] To extract a data item, the records in the auxiliary memory 30are subsequently processed and if the object identifier matches that ofthe data item to be extracted, the data piece in the main memory 31pointed to by the record is sent to the scaleable decoder 4. Since theauxiliary memory is traversed starting at the highest significance, thedata pieces will be extracted in the right order, allowing the decoder 4to progressively form a better reconstruction.

[0025] To delete a data item, the records in the auxiliary memory 30 aresubsequently processed and if the object identifier matches that of thedata item to be deleted, the significance of the record is set to a(predetermined) values that is lower than any value that can be producedby the coder, e.g. zero. Preferably, also the identifier is set to apredetermined value, e.g. zero to indicate that the memory part is notassigned to a data piece. The records in the auxiliary memory 30 arethen re-ordered to restore the ordering on significance, i.e. such thatthe records of the lowest significance are placed at the end.

[0026] The use of an auxiliary memory 30 is preferred. However, it isalso possible to omit the auxiliary memory. In that case thesignificance of a data piece and an identifier to which data item itbelongs should be stored in the main memory. Because pointers to datapieces in order of significance are in this case not available,searching the main memory 31 takes more time. To reduce searching, thedata pieces may be sorted in the main memory, at the cost of switchingmuch larger amounts of data. Also a Content Addressable Memory may beused to implement the auxiliary memory, eliminating the need for sortingand searching in the auxiliary memory. by a separate processor.Furthermore, more advanced data structures such as heaps or trees, whichare generally known, may be used for performing the administrativefunctions, as an alternative to the preferred auxiliary memory datastructure. These alternatives could be advantageous for example in asoftware implementation (or hardware when sufficient clock cycles areavailable), especially for a large number of memory parts.

[0027] In a preferred embodiment of the invention, some additionalinformation is stored for each data item. The additional information mayinclude name, type of information, color, size, etc. This additionalinformation may be stored in the main memory, e.g. together with thefirst data piece. Preferably, the additional information is stored inthe auxiliary memory, which makes it easier to retrieve this additionalinformation.

[0028] According to rate-distortion theory, as well as to scaleablecoding practice, one first has to send/store those bits that have thelargest impact on reducing the distortion in the reproduction made bythe decoder. The distortion measure that is used, depends on theapplication; e.g. one would typically use different measures for videoand audio. However, all these distortion measures have in common thatthey correspond in some way to the quality of the video/audio object asperceived by the consumer during the reproduction. Our definition ofthis measure is independent of the particular application.

[0029] The computation of a significance value of the presentapplication is discussed with reference to the method of calculationcontained in non pre-published European Patent Application 00200890.2(Our reference PHNL000110). This method was designed to optimize theoverall rate-distortion performance of the storage, i.e. to guaranteethe lowest overall distortion of the stored media items. The method useda differential distortion measure associated with individual data piecesof a data item. In particular, the significance was computed by theencoder, and was formally defined as: $\begin{matrix}{S_{i,k} = \frac{D_{i,{k - 1}} - D_{i,k}}{R_{i,k} - R_{i,{k - 1}}}} & (1)\end{matrix}$

[0030] where the index i refers to a given object and the index kindicates the number of encoded blocks that are available at thedecoder. D_(i, k-1) thus represents a distortion in the reproductionwhen k-1 subsequent blocks have been received and D_(i, k) representsthe distortion in the reproduction after k blocks have been received.R_(i, k-1) represents a rate, i.e. the total number of bits used, fork-1 subsequent blocks and R_(i, k) represents the number of bits usedfor k blocks. D_(i, 0) is the initial distortion at the decoder when nobits have been transmitted and R_(i, 0) is the number of bits when nobits have been sent, i.e. R_(i, 0)=0. When all blocks in the main memoryhave the same size, division by block size is a constant division and ispreferably left out:

{tilde over (S)} _(i,k) =D _(i,k-1−D) _(i,k)   (2)

[0031] Thus the significance value represents the reduction indistortion achieved by adding a k^(th) block. This method of calculationof significance maximizes the overall rate-distortion performance of thestorage medium to guarantee the lowest overall total distortion of thestored data items.

[0032] An embodiment of the present invention uses an alternativecomputation method for significance, in which the significance value foreach block is based on or representative of the total or absolutedistortion of a media item after decoding all of the data blocks up toand including the current data block. Thus given any distortion measurethat can be computed by the encoder, the significance may be formallydefined as:

S_(i,k)=D_(i,k)   (3)

[0033] where the index i refers to a given object and the index kindicates the number of encoded blocks that are available at thedecoder. D_(i, k) thus represents a distortion in the reproduction whenk subsequent blocks have been received and D_(i, 0) is the initialdistortion at the decoder when no bits have been transmitted.

[0034] An advantageous embodiment of the invention, which maximizes theminimum item quality, concerns a simple modification of the blocksignificance values. The minimum image quality is important, next to theaverage quality, since it gives the user a guaranteed quality for eachindividual image. Furthermore, the overall image qualityimpression/satisfaction is often determined by the lowest-quality image,since artefacts will be first or most visible in this particular image.Instead of always reducing the quality of the media item that currentlyhas the highest quality, a “look-ahead” of one quality reduction step isemployed: at each time the media item will be reduced in quality thathas the highest remaining quality after the next data block is removedfrom it. By taking into account the quality loss caused by removing thenext data block, this embodiment allows better control of the individualitem quality and guarantees the highest minimum item quality. Basically,the significance value for each block is the resulting absolutedistortion of the media item when that block is removed. All operationson the memory remain unchanged. In this embodiment the significance canbe defined as:

S_(i,k)=D_(i,k-1)   (4)

[0035] Although no further modifications are required for implementingthe new strategy, this implementation does not allow to directly obtainthe actual quality for each item, since computing the quality requiresthe significance value of the “next” block for each item, but theseblocks have either already been overwritten or don't even exist in casethe item is completely stored. In fact, compared to the distortionvalues used by the strategy of the previous embodiment, the currentsignificance values have been “shifted down” one position to the nextblock; therefore, at the end of the chain the last distortion value ismissing. To solve this issue (which is required at least for researchpurposes, e.g. to produce the PSNR values), it would thus suffice tostore and update during the memory operations the “last” distortionvalue for each item. Alternatively, instead of making thesemodifications to the memory operations, one can also store theadditional “missing” distortion value for each block. This approach issomewhat less efficient in terms of memory usage (although theadditionally required memory is negligible compared to the actualcompressed data memory), but can be implemented with minor modificationsof the available software. Therefore, this approach was taken in ourimplementation.

[0036] Another approach is the use of the total or absolute distortionof the data item up to and including said data piece in accordance withequation (3) and to check not the significance S_(i,k)=D_(i,k) of thecurrent data piece to be removed, but the significanceS_(i,k-1)=D_(i,k-1) of the next data piece (the piece with the lowestdistortion but one) of the same data item in order to determine thequality of the data item after removal of the data piece with the lowestabsolute distortion (the least significant block of the given dataitem). In this approach, the actual quality of each item remainsavailable.

[0037] An example of a distortion measure, which is often used inimages/video, is the quadratic error measure, which is the sum of thesquared pixel value differences between an original image and itsreproduction. This quadratic error measure may be used as a distortionmeasure in the present invention. After encoding block k, the encoder,in principle, has to recompute the error for each pixel of the image inorder to obtain D_(k), but in practice some simplifications/shortcuts ofthis computation are usually possible.

[0038] In a practical embodiment, a number is added to the significanceof a first data piece of each data item number after reception to makesure that the first data piece of each data item can normally not beoverwritten. This has the advantage that for each data item, one datapiece normally always remains in the memory, which makes it possible tomake a rough representation of the data item, such as a ‘thumb-nail’image. For example, to prevent the first block of an item to beoverwritten, its significance is set to “infinite”, i.e. a constantvalue larger than any distortion value that can occur for the otherblocks in the memory. In a practical embodiment, the value of 2³¹ isused for the constant and the distortion values are the absolute squarederrors. The constant 2³¹ is sufficiently large for images of dimensions768×512 pixels, but should be increased for larger image dimensions(e.g. to 2³⁹ or 2⁴⁷). Instead of increasing the constant, one could alsoscale down the distortion values (by dividing them by another constantvalue, e.g. 2⁸ or 2¹⁶).

[0039] In EP00200890.2, a difficulty arose if the sequence of datapieces of a scalably coded data item, did not have a strictly decreasingdistortion difference. In practice it required a processing of the blocksignificance values to ensure that the blocks occurring later in the bitstring always had a lower distortion reduction than the earlier blocks(i.e. to ensure a convex practical rate-distortion curve). Thisconvexity constraint is not required for the present invention, since itonly requires that the total media item distortion decreases for eachadditional block in a sequence.

[0040] In an optional embodiment, a combination of the method of thepresent invention and that of EP00200890.2 is provided in which thesignificance value is a weighted sum of the distortion difference valueof EP00200890.2 (described above) and the total or absolute distortionvalue of the present application.

[0041] It is possible to jointly store different types of objects, e.g.both audio and video, in the memory, using appropriately weightedperceptual significance measures and/or appropriately selected blocksizes.

[0042] Advantageous use of the invention is made in applications using adevice of fixed storage capacity for storing compressible data, such asvideo, images, audio, speech, etc. In which an unknown/variable amountof data has to be stored and/or for which substantially equal quality ismaintained between data items, whilst attempting to ensure that dataitems have the highest possible data quality at all times.

EXAMPLES OF ADVANTAGEOUS EMBODIMENTS ARE

[0043] Digital Still Camera. A first picture may be stored at very highquality. Subsequent pictures are stored in the fixed memory, whilereplacing some of the data of the previous pictures. Depending on thetotal number of pictures in the memory, a certain quality of thepictures is achieved. The more pictures, the lower the average quality.In a doorbell application, a picture may be taken every time someonepushes a button of a doorbell.

[0044] Answering Machine/Voice Mail. An audio data item is stored foreach person that calls and leaves a message. In case the memory iscompletely used, new audio items may be stored by reducing the qualityof the already stored audio items.

[0045] Video Recorder. A new feature offered in a video recorder is,e.g., variable recording time. One application thereof is the ability tostore more relevant programs in a lower quality instead of not recordinga program because a storage capacity of a tape or disc is entirely used.

[0046] Picture-In-Picture (PIP) Replay. A user indicates the duration ofa PIP Replay of a certain program, e.g. by pressing a button whilewatching TV. Because a give PIP-memory has a fixed capacity, the usermay exchange quality and duration of the PIP Replay.

[0047] Buffer control for embedded image memory. The whole image issubdivided into parts that are individually coded using a scaleablecoding technique. The individual parts are then split up into blocks andput into the main memory. The advantage of this new approach is that asmaller memory is required for the same image quality or a better imagequality is obtained for the same memory size. [The traditional approachis to first encode (and store) the whole image, before combining theindividual parts. Alternatively, a fixed number of bits is assigned toeach part of the image, but this is a sub-optimal approach.]

[0048] All these embodiments are preferably provided with some kind ofquality indicator to the user.

[0049] It should be noted that the above-mentioned embodimentsillustrate rather than limit the invention, and that those skilled inthe art will be able to design many alternative embodiments withoutdeparting from the scope of the appended claims. In the claims, anyreference signs placed between parentheses shall not be construed aslimiting the claim. This word ‘comprising’ does not exclude the presenceof other elements or steps than those listed in a claim. The inventioncan be implemented by means of hardware comprising several distinctelements, and by means of a suitably programmed computer. In a deviceclaim enumerating several means, several of these means can be embodiedby one and the same item of hardware. The mere fact that certainmeasures are recited in mutually different dependent claims does notindicate that a combination of these measures cannot be used toadvantage.

[0050] In summary, storing of data items in a memory is provided whereinthe data items are divided into successive data pieces of decreasingsignificance, and the data pieces are stored in respective parts of thememory, and when applying a data piece to the memory in case allcandidate memory parts are assigned to other data pieces: if thesignificance of the applied data piece is lower than the lowestsignificance of the other data pieces, discarding the applied datapiece; if the significance of the applied data piece is higher than thelowest significance, storing the applied data piece in one of thecandidate memory parts at expense of a given other data piece which hasa lower significance that the significance of the applied data piece.Preferably for each data piece a record is kept comprising asignificance of the data piece and the data item to which the data piecebelongs. The data items are preferably coded by a scaleable codingmechanism. The scaleable bit-stream is cut into data pieces of the samesizes as those available in the memory. Next, for each of these smallerpieces, a significance value is calculated. The significance value iscalculated as a total or absolute distortion value representing thetotal distortion for the successive blocks of a data item up to and/ orincluding the current block. The significance is used to compare thedata pieces of the new item with the data pieces that are alreadystored. It can easily be understood that the data pieces produced bysplitting the scaleable bit-stream have the property of decreasingsignificance, since the scaleable coding methods first produce the mostsignificant bits. The data pieces of a new item are then compared to thedata pieces already present in the memory. When the new data pieces havea higher significance, previously stored data pieces are overwritten.Advantageous use of the invention is made in applications using a deviceof fixed storage capacity for storing a flexible number of compressibledata items, such as video, images, audio, speech.

1. A method of storing data items in a memory (3), the method comprisingthe steps of: dividing each data item into successive data pieces ofdecreasing significance; storing the data pieces in respective parts ofthe memory (31); and when applying a data piece to the memory (31) incase all candidate memory parts are assigned to other data pieces; ifthe significance of the applied data piece is lower than a lowestsignificance of the other data pieces, discarding the applied datapiece; and if the significance of the applied data piece is higher thanthe lowest significance of the other data pieces, storing the applieddata piece in one of the candidate memory parts at expense of a givenother data piece which has a lower significance than the significance ofthe applied data piece, wherein the significance of each data piece isbased on a total or absolute distortion of the data item at that datapiece.
 2. A method as claimed in claim 1, wherein the total or absolutedistortion is the total or absolute distortion of the data item up toand including that data piece.
 3. A method as claimed in claim 1,wherein the total or absolute distortion is the total or absolutedistortion of the data item up to but not including that data piece. 4.A method as claimed in claim 1, wherein for each data piece a record(30) is kept, comprising the significance (S) of the data piece and thedata item (I) to which the data piece belongs.
 5. A method as claimed inclaim 4, wherein the data items are stored in a main memory (31) and therecord is kept in an auxiliary memory (30), each record furthercomprising a pointer (P) which indicates a position in the main memory(31) in which a given data piece is stored.
 6. A method as claimed inclaim 1, wherein the significance of each data piece comprises theweighted summation of a distortion difference value for the data pieceand the total distortion of the data item at that data piece.
 7. Amethod as claimed in claim 1, wherein the significance of the first datapiece of each data item is increased.
 8. A method of reading data itemsfrom a memory (3), which data items have been divided into successivedata pieces of decreasing significance, which data pieces have beenstored in respective memory parts (31), wherein an indication of thesignificance of each data piece is available in the memory, the methodcomprising the steps of: reading for each data item the successive datapieces from the memory (31), which data pieces belong to said data item;and constructing (4) the data item from the successive data pieces,wherein the significance value of each data piece is based on a total orabsolute distortion of the data item at that data piece.
 9. Anarrangement for storing data items in a memory (3), the arrangementcomprising: means for dividing each data item into successive datapieces of decreasing significance; means for storing the data pieces inrespective parts of the memory (31); and means for applying a data pieceto the memory (31), which means are arranged for, in case all candidatememory parts are assigned to other data pieces, discarding the applieddata piece if the significance of the applied data piece is lower than alowest significance of the other data pieces; or storing the applieddata piece in one of the candidate memory parts at expense of a givenother data piece that has a lower significance that the significance ofthe applied data piece if the significance of the applied data piece ishigher than the lowest significance of the other data pieces, whereinthe significance of each data piece is based on an total or absolutedistortion of the data item at that data piece.
 10. An arrangement forreading data items from a memory (3), which data items have been dividedinto successive data pieces of decreasing significance, which datapieces have been stored in respective memory parts (31), wherein anindication of the significance of each data piece is available in thememory, the arrangement comprising: means for reading for each data itemthe successive data pieces from the memory (31), which data piecesbelong to said data item; and means for constructing (4) the data itemfrom the successive data pieces, wherein the significance value of eachdata piece is based on a total or absolute distortion of the data itemat that data piece.
 11. A storage medium (3) having stored thereon dataitems, which data items have been divided into successive data pieces ofdecreasing significance, which data pieces have been stored inrespective memory parts (31), where an indication of the significance ofeach data piece is available on the storage medium, and wherein theindication of significance, wherein the significance of each data pieceis based on a total or absolute distortion of the data item at that datapiece